String of Galaxies Puzzles Astronomers

Wide-field observations of the early Universe have turned up a strange string of galaxies 300 million light-years long that defy current theories about the evolution of the Universe shortly after the Big Bang. The astronomers who discovered the string of galaxies, which are more than 10 billion light-years away, compared it to supercomputer simulations of the early Universe, which wasn’t able to reproduce strings this large this early. The next step of this research will be to map an area of the sky ten times as large to get a better idea of the large scale structure of the Universe.

Wide-field telescope observations of the remote and therefore early Universe, looking back to a time when it was a fifth of its present age (redshift = 2.38), have revealed an enormous string of galaxies about 300 million light-years long. This new structure defies current models of how the Universe evolved, which can’t explain how a string this big could have formed so early.

The string is comparable in size to the “Great Wall” of galaxies found in the nearby Universe by Dr. John Huchra and Dr. Margaret Geller in 1989. This is the first time astronomers have been able to map an area in the early Universe big enough to reveal such a galaxy structure.

The string was discovered by Dr. Povilas Palunas (University of Texas, in Austin, Texas), Dr. Paul Francis (Australian National University, Canberra, Australia), Dr. Harry Teplitz (California Institute of Technology in Pasadena), Dr. Gerard Williger (Johns Hopkins University, Baltimore, Md.), and Dr. Bruce E. Woodgate (NASA Goddard Space Flight Center, Greenbelt, Md.). The initial observations were made with the 4-m (159-inch) Blanco Telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile, and confirmed with the 3.9-m (154-inch) Anglo-Australian Telescope at Siding Spring Observatory in eastern Australia. The team presents its finding today at the American Astronomical Society meeting in Atlanta, Georgia, and a paper describing this work will appear in the Astrophysical Journal in February.

The string lies 10,800 million light-years away in the direction of the southern constellation Grus (the Crane). The distance light travels in a year, almost six trillion miles or 9.5 trillion km., is one light-year, so we see the string as it appeared 10.8 billion years ago. It is at least 300 million light-years long and about 50 million light-years wide. (Refer to Movie 1 and Images 3 and 4 for an artist’s concept of the string.) The astronomers have detected 37 galaxies and one quasar in the string, but “there are almost certainly far more than this,” said Palunas. “The string probably contains many thousands of galaxies.” (Refer to Image 1 for an artist’s concept of these galaxies, and to Image 5 for a plot of their locations on the sky.)

“We are seeing this string as it was when the Universe was only a fifth of its present age,” said Woodgate. “That is, we are looking back four-fifths of the way to the beginning of the Universe as a result of the Big Bang.”

The team compared their observations to supercomputer simulations of the early Universe, which could not reproduce strings this large. “The simulations tell us that you cannot take the matter in the early Universe and line it up in strings this large,” said Francis. “There simply hasn’t been enough time since the Big Bang for it to form structures this colossal”.

“Our best guess right now is that it’s a tip-of-the-iceberg effect,” he said. “All we are seeing is the brightest few galaxies. That’s probably far less than 1% of what’s really out there, most of which is the mysterious invisible dark matter. It could be that the dark matter is not arranged in the same way as the galaxies we are seeing.” Recently, evidence has accumulated for the presence of dark matter in the Universe, an invisible form of matter only detectable by the gravitational pull it exerts on ordinary matter (and light). There are many possibilities for what dark matter might be, but its true nature is currently unknown.

In recent years, Francis explained, it had been found that in the local Universe, dark matter is distributed on large scales in very much the same way the galaxies are, rather than being more clumpy, or less. But go back 10 billion years and it could be a very different story. Galaxies probably form in the center of dark matter clouds. But in the early Universe, most galaxies had not yet formed, and most dark matter clouds will not yet contain a galaxy.

“To explain our results,” said Francis, “the dark matter clouds that lie in strings must have formed galaxies, while the dark matter clouds elsewhere have not done so. We’ve no idea why this happened – it’s not what the models predict.”

To follow up this research, the astronomers say, the next step is to map an area of sky ten times larger, to get a better idea of the large-scale structure. Several such surveys are currently under way. The research was funded by NASA and the Australian National University.